1. Field of the Invention
This invention relates to a rotatable member for fixing and a fixing device for application to an image forming apparatus such as a copying apparatus or a laser beam printer
2. Related Background Art
A fixing devices in an image forming apparatus of the electrophotographic type such as a copying apparatus or a laser beam printer is such that a recording medium having a toner image transferred thereto is nipped and conveyed by a pair of rollers of the fixing device and is heated and pressed to thereby fix the toner image on the recording medium and make it into a permanent image.
The rollers used in such fixing device are coated with a material such as fluorine resin or silicone rubber which is good in parting property and rich in heat resisting property and wear resistance to thereby form the outer layer thereof.
Particularly where the image forming apparatus of the electrophotographic type is a color copying apparatus which attaches importance to the quality of image, there is the tendency that use is made of fixing rollers having their surfaces coated with silicone rubber.
However, silicone rubber only is low in strength and poses a problem in the use as rollers and therefore, reinforcing silica powder is mixed with it to thereby improve the physical strength thereof.
However, when silicone rubber is reinforced by inorganic powder, 10 to 40 percent by weight, practically 20 to 30 percent by weight of inorganic powder must be mixed with silicone rubber, and when a roller is made of thus reinforced silicone rubber, the problem has come out in which although physical strength is provided, the parting property of toner is extremely reduced and the life of the roller becomes shorter.
In order to suppress such reduction in the parting property of toner, there is an example in which resin-like polyorganosiloxane as a reinforcing agent is mixed with silicone rubber to thereby improve physical strength and the parting property of toner (Japanese Laid-Open Patent Application No. 5-214250).
In recent years, however, with the spread of color copying apparatuses, a tendency toward higher copying speeds has become remarkable and the set temperature for fixing has become higher. Further, with a requirement for a reduction in running cost, a longer life of fixing rollers has become desired. When the set temperature for fixing is thus relatively high, particularly 170.degree. C. or higher and the roller is used for a long time, the problem has become conspicuous that the fixing and pressing roller using as its surface layer the added type silicone rubber having the aforementioned reinforcing resin-like polyorganosiloxane mixed therewith becomes reduced in its strength by thermal deterioration and the life of the roller becomes remarkably shorter.
Particularly, when a color both-side copying machine is considered, toner comes into contact also with the pressing roller and a high parting property is required also of the pressing roller and therefore, it is preferable that the added type silicone rubber having the aforementioned reinforcing resin-like polyorganosiloxane mixed therewith be used as the surface layer, but a member for removing any excess of silicone oil imparted to the fixing roller or for removing any toner and paper powder offset to the roller bears against the pressing roller, and when the strength of the roller is reduced by thermal deterioration, there has been the problem that when they frictionally contact with the pressing roller, they impart injuries to the pressing roller, which thus becomes shorter in life than the fixing roller.